Colored images can be obtained by reacting an electron donating colorless dye (hereinafter, is referred to as color former) and an electron accepting compound (hereinafter, is referred to as color developer) in the presence of heat energy. An example of a means to obtain such an image is disclosed in U.S. Pat. No. 3,539,375 and Japanese Patent Publication No. 4160/68. Heat-sensitive recording sheets utilizing the same basic principles may be very useful as recording papers for facsimile machines.
The speed of facsimile recording machines and the speed of their print out with heat-sensitive elements have increased substantially making it necessary to have heat-sensitive recording materials which respond in a shorter period of time, i.e., heat-sensitive materials with highly sensitized recording sensitivity.
The term "recording sensitivity" as used herein refers to the relationship of heat energy applied to a heat-sensitive recording layer and the density of the image formed. A material has high sensitivity if the heat-sensitive recording material produces a high density image by applying a small amount of energy. A material has low sensitivity if the heat-sensitive recording material requires the application of a large amount of energy to obtain a sufficiently dense image.
There are shown methods of increasing recording sensitivity of heat-sensitive recording materials by incorporating various materials in the heat-sensitive color forming layers. The incorporation of sensitivity-improving agents is proposed in Japanese Patent Application (OPI) Nos. 19231/73, 48751/78, 62189/81, etc. (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). The recording sensitivity may be increased by using a bisphenol as a color developer, or by the addition of a straight chain fatty acid amide as a third material.
Sensitivity can only be increased to a limited extent by using a straight chain fatty acid amide as a sensitizing agent. Therefore, market requirements cannot be satisfied by the mere addition of the straight chain fatty acid amide.